Visual agnosia: inability to understand visual stimuli

I had stopped at a flower shop on my way to his apartment and bought a slightly extravagant red rose for my lapel buttonhole. I took it off and gave it to him. He took it as a botanist or morphologist who is given a specimen, not as a person who is given a flower.

- “About six inches in length. A rolled red shape with a green linear addition. "

-"Yes. And what do you think you are? "

- “It is not easy to say. It lacks the simple symmetry of geometric shapes, although it may have its own superior symmetry... it could be an inflorescence or a flower "

P. it acted exactly as a machine acts. It was not just that he showed the same indifference that a computer towards the visual world but that built the world as a computer builds it, through distinctive features and relationships schematic.

(...)

I start today's entry with this excerpt from a book by Oliver sacks ("The man who mistook his wife for a hat") in which a case of visual agnosia, which leads the protagonist of the story to a disintegrated vision of the world and to different situations that, although comic, result in a serious visual recognition problem.

Visual agnosia: definition and explanation

With sight being our main sense, we are always shocked and shocked by reading alterations of something as basic as perception. The brain, through its main window to the world –the eyes–, it shows us a simple and orderly image of the world around us.

This creation made by our nervous system is shared, to a greater or lesser extent, by almost everyone. The bases of everything we call reality are in the light that hits our retinas and travels through the optic nerve in the form of a nerve impulse, to make synapses in the geniculate nucleus. of the thalamus - a structure that we could consider a kind of brain toll in which a large number of synapses are made - until we reach our primary visual cortex in the lobe occipital. But it would be a mistake to believe that this circuit, these three synapses, are what give meaning to the world in which we live. What prevents us from living in a chaotic or fragmented world, as in the case of P., is the function of gnosis.

Gnosis, from the Latin knowledge, refers to the ability to recognize objects, people, faces, spaces, etc. In addition, it is also the faculty that offers us a global and united perception of reality and not schematic or "by parts". Therefore, visual agnosia is the loss of this ability. To better understand this process, we will talk about the two main brain pathways that participate in this function. We will also talk about the types of agnosia most frequently described in the bibliography.

Visual perception: the path of what and where

As we said, the information from the retina reaches our primary visual cortex after having made synapses in the thalamus. But the primary visual cortex is not in itself informative when it comes to recognition. It only processes the physical characteristics of what the retina perceives. That is: light, contrast, visual field, visual acuity, etc.

Thus, the primary visual cortex, Brodman's area 17, has only raw information. It does not tell us that we see a beautiful sunset or a dry leaf. Then, What will it take to recognize an object?

Recognizing objects, faces, places ...

In the first place, we must be able to see the object in question, making those three synapses with in order to capture the physical information of the light that first hits the object and then our retina. In second place, we must integrate all this information to perceive it as a whole. Finally, we will have to rescue from our memory the memory of that object already present in our memories and its name.

As we can see, this implies more than one source of information. In the brain, the cortex that is in charge of relating different types of information is called the associative cortex. To carry out the steps we have described we will need an associative cortex. So the brain will need more synapses, and this is when the what and where pathways come into play.

1. ID

The what pathway, or ventral pathway, is directed toward the temporal lobe and is responsible for the recognition and identification of objects. It is the way that, if for example we see in the middle of the desert a green, large and thorny thing helps us to identify it as a cactus and not as the Hulk.

It is not surprising that this pathway is located in the temporal lobe if we think that this is the main one in charge of memory functions. Therefore the way of what They are nerve projections that link the information in our retina with that in our memory. It is the synthesis of optical and limbic information.

2. Location

The way of where, or via dorsal, projects to the parietal lobe. It is the way responsible for locating objects in space; perceive their movement and trajectory, and relate their location to each other. Therefore, it is the way that allows us to direct our movements effectively in a given space.

They are the neurons that allow us to follow with our eyes the direction taken by a tennis ball that is hit from one field to another. It is also the way that allows us to send a letter to a mailbox without making mistakes.

Different neurological disorders –infarcts, head injuries, infections, tumors, etc.– may affect these pathways with the expected deficits depending on the affected region. As usual, these brain regions will not only be affected if their cortex is damaged, but also if the fibers that connect these areas with the visual cortex are affected primary.

Aperceptive visual agnosia

In this type of agnosia components of perception fail, and consequently, there is no recognition. Perception is the faculty that integrates the physical characteristics of an object so that we can capture them as a three-dimensional whole.

In apperceptive visual agnosia this integration is severely impaired and the patient shows deficits even in the recognition of the simplest forms. These patients, when faced with the drawing of a hammer, will not be able to recognize it as a hammer. Nor will they know how to copy it or match it with another drawing of the same hammer. Despite everything, visual acuity is normal, as well as the perception of light, darkness, etc. In fact, patients can even avoid obstacles when walking. However, the consequences for the patient are so dire that functionally they tend to be almost blind with serious problems in their level of independence.

Some authors, in a very timely manner, have paraphrased Saramago "there are blind people who cannot see, and blind people who seeing they cannot see." The case of a patient with apperceptive agnosia would be the second. These patients can recognize the object through another sensory modality such as touch - sometimes by touching the different parts of the object in question - or with contextual clues or descriptions of the examiner. In addition, these types of actions by the examiner help to make a differential diagnosis and rule out that the anomie - inability to say the name of what is seen - is not due to a language deficit, for example.

It is a rare type of agnosia and has been described more frequently after bilateral infarcts of the regions of the posterior arteries, carbon monoxide poisoning and in the posterior variant of Alzheimer's disease. So that, it is produced by pathologies that affect occipitotemporal regions.

Associative visual agnosia

In this type of agnosia, in addition to visual acuity, perception of color, light, contrast... perception is also preserved. However, despite normal perception, recognition is affected. As in the previous case, before the drawing of a hammer, the subject will not know that it is a hammer, but in this case he can match it with another drawing of a hammer. You can even copy the drawing or describe the object.

They may identify the drawing due to one of the details of the depicted object. As a general rule, objects are harder to identify than real ones, possibly due to a contextual factor. Again the rest of sensory modalities can help its recognition.

Associative agnosia appears to be due to the disconnect between the visual and limbic systems. The substrate may be the bilateral lesion of the white matter (inferior longitudinal fascicle) from the cortex occipital associative to the middle temporal lobe, which involves a disconnection of the visual systems and memory. That is why this agnosia is also called amnesic agnosia. The causes are similar to the case of apperceptive agnosia.

Other types of agnosia

There are many more types of agnosia and perception disorders. Below I will cite some of them. I'm just going to make a little definition to identify the disorder,

1. Achromatopsia

It is the inability to distinguish colors. Patients who suffer from it see the world in shades of gray. A bilateral lesion of the occipitotemporal region appears secondarily. There are very few registered cases. If the injury is unilateral it will not cause symptoms. I highly recommend reading "Anthropologist on Mars”In which the story of a case of achromatopsia is narrated. Also, reading Oliver Sacks is always a pleasure. I show you a fragment of said case which will be much more explanatory of the disorder than my definition:

"Mr. I. he could hardly bear the way people looked now ("like gray animated statues"), and neither could his own appearance in the mirror: he avoided social life, and sexual relations seemed impossible: he saw people's flesh, his wife's flesh, his own flesh, a gray abominable; the "flesh color" seemed to him "rat color" [.. .] He found food unpleasant due to his dull, grayish appearance, and had to close his eyes to eat. "

2. Prosopagnosia

It is the inability to recognize faces of relatives, previously known famous people or even the face of oneself in the mirror.

The prosopagnosia it is a specific deficit in face recognition and, therefore, we must rule out other types of agnosias for diagnosis. In general, other functions such as reading are not affected.

They can also estimate whether they are human or primate faces and even recognize the emotional expression of the face in question. It should be noted that the deficits are more evident when photographs are recognized than when the person in question is seen, since there will be other contextual clues such as the movement of the latter. Also very interesting is the proposal by Damasio et al (1990) who would consider that prosopagnosia would not be so much a failure in face recognition, but rather the inability to identify individuality within a set of Similar.

3. Acinetopsia

It is the inability to perceive moving objects. It is frequently due to posterior occipitoparietal lesions. The first case of acinetopsia was described in 1983 in a 43-year-old woman who had suffered several bilateral cerebrovascular infarcts. The deficits seriously affected their level of independence. For example, he needed to touch the rim of the cup to know when to pour coffee.

Some conclusions

I think it is not necessary to justify how basic the function of gnosis is for our lives. In a way, our consciousness depends on what we see and the reality that makes up our brain. This "reality", manufactured by our circuits, is possibly far from what is reality as such. Let's think for a moment: when we see how someone speaks, what we see and what we hear generally has a synchronicity. That is, if a friend speaks to us, we should not see that he first moves his mouth and then we listen to the sound, as if it were a badly dubbed movie. But, instead, the speed of light and the speed of sound are very different.

The brain, in some way, integrates reality so that we understand it in an orderly and logical way. When this evil Cartesian genius fails the world can take on a chaotic and aberrant tone. As the fragmented world of P. or the world absent of color of I. But is their world more unreal than ours? I think not, we all live somehow deceived by our brain. As if we were in the Matrix. A Matrix created by ourselves.

Patients like P. I heard. they have contracted pathologies that have made them distance themselves from the "reality" that we are used to sharing with other human beings. Although these specific cases had happy endings characterized by self-improvement, in the usual Oliver Sacks vein, it should be noted that not all cases are equally beautiful. Neurologists and neuropsychologists only see clinical manifestations of these pathologies and, therefore Unfortunately, on many occasions when faced with these cases we are forced to adopt an attitude of "voyeur". Namely, many times we cannot do much more than follow the case and see how it evolves.

Currently, pharmacological therapies for neurodegenerative disorders are of very limited use. Science must develop new drugs. But neuropsychologists must develop new non-pharmacological therapies beyond classical cognitive stimulation. Centers such as the Guttmann Institute, specialists in neurorehabilitation, are making a great effort and dedication in this. My subjective opinion is that perhaps the new virtual reality therapies will mark the 21st century of neuropsychology. In any case, we must work on this or other options and not settle for just the diagnosis.

• Text corrected and edited by Frederic Muniente Peix

Bibliographic references:

Books in which cases of Agnosia are narrated and I highly recommend reading them:

• Luriia, A., Lemos Giráldez, S., & Fernández-Valdés Roig-Gironella, J. (2010). Lost and recovered world. Oviedo: Krk Ediciones.
• Sacks, O. (2010). The man who mistook his wife for a hat. Barcelona: Anagram.
• Sacks, O. An anthropologist on Mars. Barcelona: Anagram

Textbooks:

• Arnedo A, Bembire J, Tiviño M (2012). Neuropsychology through clinical cases. Madrid: Editorial Médica Panamericana.
• Junqué C (2014). Neuropsychology Manual. Barcelona: Synthesis

Articles:

• Álvarez, R. & Masjuan, J. (2016). Visual agnosias. Revista Clínica Española, 216 (2), 85-91. http://dx.doi.org/10.1016/j.rce.2015.07.009

I highly recommend this article above. It is very well explained and is very clear and concise.

• Barton, J. (1998). Higher cortical visual function. Current Opinion In Ophthalmology, 9 (6), 40-45. http://dx.doi.org/10.1097/00055735-199812000-00007
• Barton, J., Hanif, H., & Ashraf, S. (2009). Relating visual to verbal semantic knowledge: the evaluation of object recognition in prosopagnosia. Brain, 132 (12), 3456-3466. http://dx.doi.org/10.1093/brain/awp252
• Bouvier, S. (2005). Behavioral Deficits and Cortical Damage Loci in Cerebral Achromatopsia. Cerebral Cortex, 16 (2), 183-191. http://dx.doi.org/10.1093/cercor/bhi096
• Naccache, L. (2015). Visual consciousness explained by its impairments. Current Opinion In Neurology, 28 (1), 45-50. http://dx.doi.org/10.1097/wco.0000000000000158
• Riddoch, M. (1990). M.J. Farah, Visual agnosia: Disorders of object recognition and what they tell us about normal vision. Biological Psychology, 31 (3), 299-303. http://dx.doi.org/10.1016/0301-0511(90)90068-8
• Zeki, S. (1991). Cerebral Akinetopsia A Review. Brain, 114 (4), 2021-2021. http://dx.doi.org/10.1093/brain/114.4.2021